When used as a fixer fixing an developing agent on a recording medium in copying machines, printers, and other electrophotographic image forming apparatuses, heaters heating a medium (heated material) under heat and pressure are typically arranged to operate according to heat roller fixing.
A fixer arranged to operate according to heat roller fixing has a pair of rollers (heat and pressure rollers) disposed to press each other. Inside the roller (heat roller) in contact with the toner image side of a recording medium (recording paper) is provided heating means, such as a halogen heater.
The heat roller is heated to a predetermined temperature (fixing temperature) by the heating means. Then, a recording medium carrying an unfixed toner image is passed through a pressure application section (fix nip section) of the heat and pressure rollers where the toner image is fixed under heat and pressure.
A disadvantage of heat roller fixing is that it takes a long time (warm-up time) for the heat roller to reach the fixing temperature after the onset of its heating. For convenience, the heat roller needs to be pre-heated when in standby. Power consumption in warm-up and standby is considerable.
To address the problems, fixers have been proposed recently which implement thin heat roller fixing. Japanese unexamined patent application 9-244448 (Tokukaihei 9-244448/1997; published on Sep. 19, 1997), for example, discloses such a fixer.
The thin-heat-roller-fixing fixer includes a heat roller with core metal having a reduced thickness for a reduced thermal capacity. The warm-up time is thus reduced, which reduces the warm-up and standby power consumption.
However, in thin heat roller fixing, the heat roller can be reduced in thickness relatively easily in low-to-medium speed apparatuses (e.g., capable of making less than 50 A4 copies per minute when the recording paper is fed in landscape orientation), but only with difficulty in high speed apparatuses (e.g., capable of making 50 or more A4 copies per minute when the recording paper is fed in landscape orientation).
These are reasons why high speed apparatuses have longer warm-up time (typically 3 minutes or longer) and greater power consumption. Referring to FIG. 16, the following will describe difficulties in incorporating a thin heat roller in high speed apparatuses.
FIG. 16 is a graph showing relationships between a nip transit time, a heat roller temperature (roller temperatures), and a pressure applied to recording paper in the fix nip section (surface pressures) in conventional low-to-medium and high speed apparatuses.
The nip transit time is also called the duel time. It is the width of the fix nip section divided by the fixing rate (transport speed of recording paper) and represents the time taken for any given point on recording paper to pass through the fix nip section. A greater copy rate normally means a shorter nip transit time.
It would be understood from FIG. 16 that the low-to-medium speed apparatus uses higher fix roller temperatures to achieve a greater copy rate, i.e., to compensate for a shorter nip transit time and by doing so, applies an adequate quantity of heat to the recording paper to ensure sufficient fixing performance. The surface pressure is therefore substantially constant regardless of nip transit time.
The high speed apparatus must work at a greater fixing rate than the low-to-medium speed apparatus, resulting in a nip transit time of 23 ms (2.3×10−2 sec.) or less. Meanwhile, the always-on temperature for the heat roller is limited (for example, to 200° C.) in an ordinary situation. In view of heat resistance issues of the heat roller, the heat roller temperature cannot be raised exceeding the limit temperature. The high speed apparatus therefore ensures sufficient fixing performance by means of a high surface pressure while maintaining a constant heat roller temperature (at the limit temperature).
This places a heavy load on the heat roller, which unlike in the low-to-medium speed apparatus inhibits the provision of a thin heat roller in the high speed apparatus. Thus, the warm-up time is difficult to reduce. A result is great power consumption.
In addition, the heavy load on the heat roller causes the heat roller to creep and suffers from a shortened lifetime, as well as causes the recording paper to crease and curl up.
Further, the difficulties in reducing the heat roller thickness invite increases in size of the apparatus. In addition, the heat roller will have an increased drive torque which in turn entails increased power consumption and shortened driver components lifetime.
To address the aforementioned large warm-up and standby power consumption problems, fixers implementing external roller heating (hereinafter, “external roller heating fixers”) have also been proposed recently. Japanese unexamined patent application 2000-338818 (Tokukai 2000-338818; published on Dec. 8, 2000), for example, discloses such a fixer.
An external roller heating fixer incorporates an external heat roller into the fixer implementing heat roller fixing. The external heat roller is an auxiliary heating means for the pressure roller. It contacts the pressure roller to externally heat the surface of the pressure roller.
This raises the surface temperature of the pressure roller and unlike in the fixer for heat roller fixing, enables the pressure roller to actively supply thermal energy to the recording paper. The warm-up time is thus reduce, and so is the pre-heating of the heat roller in standby, enabling reductions in power consumption.
In addition, the pressure roller actively supplying additional thermal energy to the recording paper allows reductions in the fixing load to the recording paper. This prevents the recording paper passing through the fix nip section from curling up.
Further, the reduced fixing load opens up possibilities that the fixer may be used in high speed apparatus generally regarded as requiring a heavy fixing load (e.g., 55 or more A4 copies [sheets] per minute when the recording paper is fed in landscape orientation, or a 23 millisecond or less transit time taken for any given point on recording paper to pass through the fix nip section). Thanks to the reduced fixing load, the external roller heating fixer allows the use of a heat roller which is reduced in thickness and/or diameter, hence in thermal capacity. The fixer thereby shortens the warm-up time and accordingly reduces power consumption. For these reasons, the fixer is suitably applied in the high speed apparatus field.
The pressure roller in the external roller heating fixer has however a higher surface temperature and thus dissipates more heat from the surface than the counterpart in the fixer for heat roller fixing. The external heat roller also dissipates heat from the surface.
Therefore, under some structural and physical conditions of the external heat roller, such as its diameter, thickness, the load it exerts to the pressure roller, and its surface temperature, the fixer dissipates too much heat, which possibly causes poor heat efficiency, higher internal temperature, and like problems. An outcome may be the opposite of what is intended in the first place: greater power consumption than the fixer for heat roller fixing.